Ab initio study of structural, bonding, and vibrational properties of AGaO 2 (A = Ag, Cu) delafossites

Abstract

We have performed an ab initio study of structural, bonding, and vibrational properties of AgGaO 2 and CuGaO 2 delafossites. The calculations have been carried out within the density functional theory and linear response formalism using ultrasoft pseudopotentials and a plane wave basis. Structural properties of AgGaO 2 and CuGaO 2 are in very good agreement with experimental data, and zone centre phonons in CuGaO 2 are in excellent agreement with experimental and previous DFT calculation. Two Raman-active modes, E g and A 1 g in AgGaO 2 are found to have frequencies at 367 cm −1 and 682 cm −1 respectively. A noticeable difference between these calculated and experimentally observed Raman-active modes have been found. Calculated Raman-active modes have been further verified using atomic-orbital basis and norm-conserving pseudopotentials within finite displacement frozen phonon technique. We also present the phonon dispersion relations, phonon density of states, pressure-coefficients, and mode-Grüneisen parameters at high symmetry points in these oxides. Our DFT calculations predict dynamical instability, possibly related to phase transition around 17 GPa in AgGaO 2 delafossite.